Composites have been widely used in the aerospace industry due to their excellent characteristics such as high specific strength, specific stiffness, good fatigue resistance and designability.
With the progress of digital aircraft manufacturing technology and the development of composite manufacturing processes, digital design and digital manufacturing are gradually becoming the core technology of composite manufacturing.
Combined with the digital manufacturing technology of composite materials, the digital inspection of composite parts has become one of the key links to control the quality of composite parts.
The traditional inspection method for composite parts is to make inspection samples, which has low precision, low efficiency and high cost.
With the development of digital measurement technology, portable coordinate measurement system, especially the emergence of laser tracker, provides the necessary technical means for the digital inspection of composite parts.
System components and measurement principle of laser tracker measurement
The laser tracker is known as a mobile CMM, it is a portable coordinate measuring system based on the spherical coordinate system, with the advantages of high measurement accuracy, real-time fast, dynamic measurement, easy to move.
The laser tracker can measure the distance to the target point and the horizontal and vertical direction deflection angle.
Its basic principle is to place a reflector on the target position, laser tracking hair out laser shot to the reflector and back to the tracking head.
As the target moves, the tracking head adjusts the beam direction to focus on the target.
At the same time, the return beam is received by the detection system and is used to measure the spatial position of the target.
In summary, the laser tracker is used to determine the spatial coordinates of a target point by measuring the position of a reflector placed on the target point.
The laser tracker can directly measure the three-dimensional coordinates of the spatial point, which are obtained in the laser tracker’s instrument coordinate system.
This coordinate system is defined as: with the center of the tracking head as the origin, with the direction of the 0 reading on the degree dial as the X-axis, with the normal upward direction of the degree dial plane as the Z-axis, and with the rules of the right-handed coordinate system to determine the Y-axis.
The instrument coordinate system established according to the above requirements is as shown in Figure 1.
Figure 1 Laser Tracker measurement schematic
When the reflector moves away from the reference position (the distance of the reference position from the center of the instrument is known) and moves in space, the laser tracker automatically tracks the reflector and records the interferometric distance value D and the angle values α and β on the verticality and horizontality dials at the same time.
Using these three observations, the spatial 3D Cartesian coordinates (x, y, z) of the point can be obtained according to equation (1).
Inspection of composite parts
An aircraft composite reinforcing rib was manufactured by traditional analog techniques.
Now with the need for digital manufacturing, a laser tracker is needed to detect deviations from the theoretical position of this part.
The laser tracker measurement system used in this paper is a Leica AT901-LR, with a measuring radius of 80m, spatial length measurement uncertainty of 15μm + 6μm/m; T-probe measuring radius of 15m and spatial length measurement uncertainty of 7μm/m.
The measurement process of the composite parts included 3 steps: the establishment of the measurement modulus, the establishment of the measurement coordinate system, the measurement and the analysis of the results.
1. Establishment of the measurement modules
Because this composite part is manufactured by traditional analog technology without digital model, so the first step must be to create a measurement model of the part before digital measurements can be made.
The process of manufacturing large composite parts is different from the traditional process of manufacturing metal parts.
Due to the characteristics of composite materials themselves, a large number of high-precision machining cannot be performed at a later stage.
Therefore, the molding of composite parts is characterized by integrated molding, later precision machining is less, and its accuracy is basically completely dependent on the precision of the molding die to ensure.
The accuracy of the forming die for the composite parts in this paper has been verified by inspection samples, so the forming die can be used as the basis for testing the composite parts.
Using the laser tracker to measure the composite molding mold, using the measurement results to establish the measurement modules.
The process of establishing the measurement modulus is:
Firstly, it measures points on the die by a laser tracker.
Then, the measurement point cloud is formed by a large number of measurement points.
Finally, a large number of the measurement point cloud are fitted to a calculation to generate a profile.
This profile can be used as the part’s measurement model because it fits the part’s profile.
2. Establishment of the measurement coordinate system
The measuring die must be measured in the same coordinate system with the composite part, so a measuring coordinate system must be established.
The established method is to set fixed measurement points around the perimeter of the die when establishing the measurement digital model, using a laser tracker to measure these points, and recording the spatial coordinate measurements.
This measurement point can be used as the reference point of the measurement coordinate system, and any measurement of the part is based on these measurement points as the original reference point.
3. Measurement of parts
After the composite part is positioned on the die, the die is removed.
A laser tracker is then used to measure the reference point, which is used to keep the part in the same coordinate system as the measurement die.
Once the coordinates are unified, the part is inspected.
Using a reflector to directly measure the part profile, the measurement software automatically compares the measured data with the theoretical data of the measurement modulus, which is not only to measure the geometric position of the part, but also to comprehensively evaluate the overall position of the state in the current coordinate system.
4. Results analysis
The factors influencing the measurement results mainly include: instrument accuracy, vibration, parts placement and other aspects.
Instrument accuracy is a fixed factor, due to the accuracy of the composite parts required for ± 1mm, which is much greater than the accuracy of the laser tracker, so the impact of the instrument accuracy is almost negligible.
Vibration can be controlled by choosing the station of the laser tracker.
Part placement becomes a major factor in the measurement results.
Since the position of the laminating part is determined by the forming die during the measurement process, while it is impossible for the parts to fit perfectly with the die in the installation process, so the placement of the parts is bound to produce a large error.
The solution is to optimize the numerical fit to the measurement results by using the part itself as a reference, which will reduce the influence of the measurement results due to the placement.
Table 1 Measurement results before and after fitting optimization
|Deviation before optimization /mm||Deviation after optimization /mm|
|measuring point 1||1.368||0.447|
|measuring point 2||1.308||0.434|
|measuring point 3||1.266||0.442|
|measuring point 4||1.165||0.391|
|measuring point 5||1.089||0.373|
|measuring point 6||0.906||0.251|
|measuring point 7||1.029||0.424|
|measuring point 8||0.905||0.401|
|measuring point 9||0.910||0.453|
|measuring point 10||0.839||0.419|
|measuring point 11||0.786||0.402|
|measuring point 12||0.820||0.480|
|measuring point 13||0.626||0.342|
|measuring point 14||0.668||0.448|
|measuring point 15||0.680||0.499|
|measuring point 16||0.610||0.482|
From Table 1, the pre-optimization measurement results deviate significantly from the theoretical values, but these values do not represent the true condition of the part.
By optimizing the fit, the deviation of the measurement results is significantly reduced, eliminating errors due to placement.
And the optimized results can reflect the real condition of the part.
Digital design and manufacturing technology is an inevitable trend in the development of composite forming die design and manufacturing technology, rapid digital inspection.
It not only solves the drawbacks of traditional analog quantity transfer but also provides an effective means of quality control for composite manufacturing.